1. Technical Field
The technical field relates generally to electroacoustic transducers and more particularly to an electro-dynamic projector capable of absorbing high power inputs for generating substantial underwater acoustic energy over a broad frequency range at a varying depths.
2. Description of the Problem
The predominant types of electroacoustic projectors/transducers in contemporary use for generating sound for propagation through water are: piezoelectric; magnetostriction; hydraulic acoustic; and electro-dynamic. Piezoelectric transducers are particularly common due to their simplicity, electrical efficiency and low distortion within their operative band width. However, piezoelectric devices are characterized by narrow resonance peaks, phase shift issues and poor damping of ring down. While the relatively high voltages and low currents at which piezoelectric devices operate contribute to their high efficiency, high voltage operation can be an issue in salt water environments due to the relatively high electrical conductivity of salt water.
Massa, in U.S. Pat. No. 4,763,307 taught underwater electro-dynamic transducers based on moving coil and moving iron principals. These transducers were combined with a variable pressure, gas filled back chamber for housing the transducer electrical circuit. The variable pressure back chamber balanced pressure in the back chamber, and behind the piston, with external ambient pressure. Pressure variation was achieved by providing a bladder which collapsed with increases in ambient pressure. The bladder communicated with the space behind the piston/diaphragm through a breather tube. This should prevent the bladder volume from functioning as a (variable frequency) tuned chamber for the diaphragm.
Moving coil and moving iron devices operate at lower voltages than piezoelectric devices which reduces electrical issues with operating in a salt water environment. However, low voltage operation entails the use of high currents. High current flow through the transducer voice coil to produce a high acoustic power output results in the generation of substantial amounts of heat from resistive losses in the system's voice coil. Massa recognized a need to sink heat from the voice coil for the moving coil design and employed heat conducting metal strips between the piston mounted voice coil and the transducer piston to transfer heat to the exposed face of the piston.
Most contemporary electro-dynamic transducers for both air and water applications use a moving coil design. In a moving coil transducer a stationary permanent magnet is positioned close to a speaker diaphragm. An electrical current carrying voice coil is glued to the diaphragm. Upon application of an alternating electric current to the coil the coil is attracted or repelled from the magnet with the changes in phase of the current. Since the diaphragm to which the coil is attached can move acoustic waves may be induced in a transmission medium, such as air or water, from diaphragm. Moving iron loudspeakers place an iron or a similar ferro-magnetic material on the speaker diaphragm and provide a stationary voice coil. Moving iron loudspeakers were common in the 1920's, but were gradually displaced for most applications in order to reduce diaphragm mass.